The contractors can't bid intelligently because they usually don't have the core competencies (and often don't even know what they are) and will be planning to outsource them. A prime reason they are brought and paid a premium is so that the client has a central focal point for outsourcing blame.

The odds that they stumble upon the right technologies and the right people are extremely slim , few companies are intellectually agile enough to possess that expertise in house. That model of a company with specialist consultancy practices (applicable in most other professions) has largely died out in IT and been replaced by outsourcers that offer primarily project management skills that reckon they can throw together a team of people to meet a need. Unless the client already largely knows the technological space they are dealing with there are way too many things that can and will go wrong. I can only assume it's done because the front end of an application can be a very effective camouflage for monstrosity and the expectations on the contractor are so low.

Again if you look to other professions this is not how they do things.